Imagine that you are in
the middle of the ocean, and you are looking all around you in every direction
and all you can see is water. It is overcast so you cannot see the sun.
How would you know which way to go unless you had a compass to tell you which
way is North?
In order to find direction, early map makers drew a small 16-pointed circle on a
map. Next, they would place an "N" to point North. These points became
known as the 16 Cardinal Points from which the winds of the Earth were thought
to blow. The drawing became known as a Wind Rose. Once the magnetic
compass was developed, the magnetic compass was placed on top of the Wind Rose
pattern. This helped insure that the nautical chart would face in the
proper direction. In time, the Wind Rose became known as a Compass Rose.
For centuries, this was
the best in accuracy for traveling the seven seas. Obviously, it is not
very accurate by today's standards. With more information, and the
improvement of spherical mathematics, it eventually became customary to give
bearings in units of degrees rather than the use of Cardinal Points.
During the early part of the 20th Century, it became accepted practice to
indicate direction (also called "heading" or "bearing") in terms of degrees
representing the degrees of a circle as measured clockwise from True North.
No matter where you stand
on Earth, you can hold a compass in your hand and it will point toward the North
Pole. Long before GPS satellites and other high-tech navigational aids,
the compass gave humans an easy and inexpensive way to orient themselves.
While you might think the compass is no longer useful thanks to our GPS
satellite system and handheld GPS indicators, you would be wrong. People
use the compass, along with a map, in the sport called Orienteering.
Besides, it is important to learn how to use a compass in the event some alien
civilization knocks out our GPS satellites!
The compass is an extremely simple
device. A magnetic compass consists of a small, lightweight magnet
balanced on a nearly frictionless pivot point. The magnet is generally
called a needle and one end of the needle is colored in some way to indicate
that it points toward north.
The reason why a compass works is this: think of the Earth
as having a gigantic bar magnet buried inside. In order for the north end of
the compass to point toward the North Pole, you have to assume that the
buried bar magnet has its south end at the North Pole. Hence, the
"opposites attract" rule of magnets causes the north end of the compass
needle to point toward the south end of the buried bar magnet, and therefore
the compass points toward the North Pole.
The bar magnet does not run exactly along the Earth's
rotational axis. It is skewed slightly off-center. This skew is called
declination, and well-made maps indicate what the declination is in
different areas because it changes depending on where you are on the planet.
The magnetic field of the Earth is relatively weak on the surface because
with Earth being almost 8,000 miles in diameter, the magnetic field has to
travel a long way to affect your compass. That is why a compass needs
to have a lightweight magnet and a frictionless bearing; otherwise, there
wouldn't be enough strength in the Earth's magnetic field to turn the
needle.
Obviously, there is no big bar magnet that runs through
the middle of the Earth. However, what really is happening is still a
theory. It is believed the Earth's core consists largely of molten
iron. At the very core, the pressure is so great that this super-hot
iron crystallizes into a solid. Convection caused by heat radiating
from the core, along with the rotation of the Earth, causes the liquid iron
to move in a rotational pattern. It is believed that these rotational
forces in the liquid iron layer lead to weak magnetic forces around the axis
of spin.
A magnetic compass has several
problems when used on moving platforms like ships and airplanes. It
must be level, and it tends to correct itself rather slowly when the
platform turns. As a result, ships and airplanes use gyroscopic
compasses. A spinning gyroscope maintains the direction it is
pointing. In a gyrocompass, this tendency is used to emulate a
magnetic compass. At the start of the trip, the axis of the
gyrocompass is pointed toward north using a magnetic compass as a reference.
A motor inside the gyrocompass keeps the gyroscope spinning, so the
gyrocompass will continue pointing toward north and will adjust itself
swiftly and accurately even if the boat is in rough seas or the plane hits
turbulence. Periodically, the gyrocompass is checked against the
magnetic compass to correct any error it might pick up.
Obviously, this is old technology.
With our Global Positioning System (GPS) satellites in orbit around the
Earth, it has become much easier to pin-point location. The GPS
consists of 24 Earth-orbiting satellites. These satellites allow any
person who owns a GPS receiver to determine his or her precise longitude,
latitude and altitude anywhere on the planet.